Vertical-cavity surface-emitting lasers (VCSELs) are semiconductor lasers in which light emission occurs perpendicularly to the surface of the semiconductor chip. Vertical-cavity surface-emitting laser diodes have a plurality of advantages over conventional edge-emitting laser diodes, such as low electrical power consumption, the possibility of directly inspecting the laser diode on the wafer, simple possibilities for coupling to a fibre optic, longitudinal single-mode spectra and the possibility of connecting the surface-emitting laser diodes to a two-dimensional matrix.
Typical vertical-cavity surface-emitting semiconductor laser diodes having an emission wavelength λ (=vacuum wavelength) have a resonator comprising at least a first distributed Bragg reflector (DBR), an active zone having a p-n junction and a second distributed Bragg reflector. Laser diodes of this type generally possess a cylindrically symmetrical structure and have, owing to their design and also the methods for the manufacture thereof, no preferred direction of polarisation of the emitted wave. There are therefore two orthogonal states with regard to the direction of polarisation of the emitted wave. In an ideal laser structure, these two states are energetically degenerate and on an equal footing for the laser operation. However, owing to the electrooptic effect, to anisotropies in the component design and to asymmetries and fluctuations in the manufacturing process, this degeneration is cancelled and the VCSEL oscillates predominantly in one polarisation mode. In most cases, the mechanism leading to a VCSEL favoring a specific mode is difficult to monitor or not obvious, resulting overall in a statistical character of the polarisation behaviour. Polarisation jumps generally limit use in polarisation-dependent optical systems. For example, jumps of this type in optical data transmission lead to increased noise. As many applications are dependent on polarisation-stable lasers as light sources, this means a significant reduction in the production yield. Although a preferred direction can be defined in some cases, cancellation of degeneration is not sufficiently strong to ensure polarisation stability under variable environmental and operating conditions. In this case, even minor alterations of these parameters can cause a change-over between the two states (pole flip).
In the past, various possibilities were studied for stabilising polarisation. In order to achieve polarisation stability of GaAs-based VCSELs, growth was successfully demonstrated on higher-indexed [311] substrates, cf. in this regard “An 850-nm InAlGaAs Strained Quantum-Well Vertical-Cavity Surface-Emitting Laser Grown on GaAs (311)B Substrate with High-Polarization Stability,” IEEE Photon. Technol. Lett., 12, 942 (2000). However, as the remaining laser properties are generally impaired and difficult growth conditions exist, in particular for InP-based semiconductor layers, this method would not appear suitable for long-wave VCSELs.
Another approach includes applying dielectric or metallic grating structures with periods in the wavelength range to the output mirror. This is described for example in the publication “Polarisation stabilisation of vertical-cavity top-surface-emitting lasers by inscription of fine metal-interlaced gratings,” Appl. Phys. Lett. 66, 2769 (1995) or DE 103 53 951 A1. Dielectric gratings used lead to interference effects, as a result of which total reflection is strengthened or weakened by the grating in a polarisation-dependent manner. A period of the corresponding grating structures must therefore be more than half a vacuum wavelength.
In addition, it is also known to apply metal/dielectric or metal/semiconductor structures with periods smaller than a wavelength of a VCSEL to one of two distributed Bragg reflectors in order to generate polarisation by birefringence. This is disclosed for example in the publication “Polarization control of vertical-cavity surface-emitting lasers using a birefringent metal/dielectric polarizer loaded on top distributed Bragg Reflector,” IEEE J. Sel. Top. Quantum. Electron. 1, 667 (1995) or JP 80 56 049 (A). In this solution, metallic-dielectric gratings are intended to serve to generate birefringence in the laser resonator. Thus, an optical resonator length or a resonance frequency of the laser resonator is intended to correspond to a reflectivity maximum of the Bragg mirror or mirrors only in one polarisation. The other polarisation is to be suppressed. However, the described experimental results show that an achievable polarisation orientation is not sufficient.
Whereas the above-described embodiments have a periodic structure for controlling polarisation outside an inner resonator region (the latter corresponds to the region between the two DBRs), JP 2005 039102 A describes a construction in which a periodic grating is located between the two DBRs. This structure consists, on the other hand, of two separate and independent components, the periodic grating not being in direct contact with one of the two DBRs. Problems that are typical in the manufacture of one-piece or monolithic diodes, such as for example diffusion, therefore do not occur in this configuration.
A further embodiment of a periodic grating is described in US 2003/0048827 A1. In this case, use is made of a monolithic construction in which a grating is epitaxially integrated into one of the two semiconductor distributed Bragg reflectors. The alternating layer system is based in this case primarily on a different doping. However, only comparatively small differences in index of refraction can be generated in this way, as a result of which the polarisation mode is not sufficiently split.